REPRINTED FROM THE 2023 HYPERBARIC INDICATIONS MANUAL 15^th Edition: Sudden Sensorineural Hearing Loss

Sudden sensorineural hearing loss (SSNHL) presents as the abrupt onset of hearing loss. Approximately 88% of SSNHL has no identifiable etiology and is termed idiopathic sudden sensorineural hearing loss (ISSHL). Hearing specialists have investigated ISSHL since the 1970s. Over the past 30 years, more than 800 articles, or one every two weeks, have been published in the English medical literature. ISSHL is the abrupt onset of hearing loss, usually unilaterally and upon wakening, that involves a hearing loss of at least 30 decibels (dB) occurring within three days over at least three contiguous frequencies. As most patients do not present with premorbid audiograms, the degree of hearing loss is usually defined by the presentation thresholds of the unaffected ear. Other associated symptoms include tinnitus, aural fullness, dizziness and vertigo. The historical incidence of ISSHL ranges from 5-20 cases/100,000 population, with approximately 4,000 new cases per annum in the United States. The true incidence is thought to be higher, as ISSHL is thought to be underreported. Interestingly, 4,000 cases annually calculate to 1.3 cases/ 100,000 in the United States; therefore, an incidence of 5-20/100,000 would translate to > 15,000 new ISSHL cases per annum in the United States. Recent literature has placed the annual ISSHL incidence in the United States as 27 cases/100,000, with a pediatric incidence of 11 cases/100,000. Other studies report that the incidence is increasing (160/100,000), especially in the elderly (77/100,000), and conclude that ISSHL is no longer rare. In 1984, Byl reviewed the literature and found the mean age of ISSHL presentation to be 46-49 years, with variation of incidence with age and an equal gender distribution. The presentation of ISSHL does not appear to have seasonal variations, uneven distributions of presentation throughout the year, or an association with upper respiratory infections, either prior to or following symptom onset. The spontaneous recovery is currently thought to be 30-60%.